Analog to digital converters (ADCs) may be used to convert a variety of types of analog input signals to digital outputs. Often, the analog signal requires some analog preprocessing before converting the analog signal to a digital signal. For example, the analog signal may first be amplified by a preamplifier. A common application for preamplifiers is to amplify low voltage level signals which are to be processed by ADCs that operate at higher voltage levels. For example, a preamplifier may be used to amplify a transducer output (a thermocouple output, strain gauge output, thermistor output, etc.) prior to processing the transducer output in the ADC. Transducer signals often are amplified because most transducers produce only low voltage outputs while the ADC may operate at a significantly higher voltage range. For example, a thermocouple may provide an output signal having a range of 2.5 mV while an ADC utilized to convert the thermocouple output into a digital signal may operate at a 2.5V full scale voltage. Therefore, a preamplifier may be utilized to amplify the transducer output prior to processing the output signal with the ADC. Because the ADC may have a relatively high noise density, the use of a preamplifier reduces the ADC's output noise when that noise is input-referred to the preamplifier input (i.e., the noise at the signal processing circuitry output is divided by the gain). However, the use of a preamplifier typically has a dynamic range drawback since improved low end dynamic range is provided at the expense of high end dynamic range. It is thus desirable to provide a preamplifier configuration which avoids high end dynamic range loss.
The amount of amplification required to be provided by the preamplifier may vary depending upon the transducer output characteristics. Thus, a programmable preamplifier which may be programmed to different gain values is desirable so that a single preamplifier may be used with a variety of input voltage signals. FIG. 1 illustrates a typical prior art analog to digital conversion system which includes an analog input 100, a programmable preamplifier 110, an ADC 112 and a digital output 104. In such a system the gain of the preamplifier 110 may be user programmable by supplying a user defined gain setting input 111 depending upon what type of analog source is utilized. The system of FIG. 1 may be implemented monolithically with a serial control port provided to receive the gain setting input. Such serial control ports generally do not operate synchronously with the ADC sampling frequency and also operate at a lower word rate than the sampling frequency.
The analog to digital conversion systems may also be configured to be coupled to a plurality of different analog sources. For example, the preamplifier input may be switchably coupled to a plurality of different transducer inputs and each transducer may have a different output voltage characteristic. In such circumstances it is desirable to adjust the preamplifier gain depending upon the signal level presented at the preamplifier input. Whether one analog source or a plurality of sources are coupled to the preamplifier, the prior art techniques generally require the user to know the range of the analog input signal and user intervention to set the gain is required.
Typical preamplifier configurations are comprised of operational amplifiers (opamp) and resistors. FIG. 1A illustrates a typical preamplifier configuration. As shown in FIG. 1A, the preamplifier 1 is comprised of an opamp 3 and resistors R1-R4. By selectively closing one of the switches Sa, Sb, and Sc, the gain of the preamplifier may be programmably set. Ideally the closed switch would provide negligible resistance and the gain at the opamp output Vopamp/Vin would be independent of the switch resistance. However, because the switch is not ideal and adds some gain error due to its resistance, the preamplifier output may chosen at the nodes Vout1, Vout2 or Vout3 so that any error caused by the switch resistance is negated. Thus, it can be shown that for equal values for resistors R1-R4 if Sa is closed Vout1/Vin=2, if Sb is closed Vout2/Vin=3, and if Sc is closed Vout3/Vin=4 (i.e., the gain equals X, where the number of resistors between Vout and the inverting input of the opamp is X-1).
Monolithic implementations of circuits such as that shown in FIG. 1A may have gain drifts with temperature in excess of 4 or 5 ppm (parts per million) per degree Celsius. The predominate mechanism producing such drift may be the ratio drift of the gain setting resistor strings. One approach to minimize the ratio drift of the resistor string is to remove from resistor string contacts from the resistor string current path. Such a technique is shown in U.S. Pat. No. 5,319,319 to Kerth, the disclosure of which is incorporated herein by reference. The preamplifier of U.S. Pat. No. 5,319,319 is not, however, easily adapted to provide a preamplifier configuration which avoids high end dynamic range loss as discussed above.
A variety of types of analog to digital converters ("ADCs") are commonly employed for converting analog input signals to a digital output. One type of ADC is a successive approximation ADC. A switched capacitor array is one type of successive approximation ADC. Switched capacitor array ADCs are known in the art as shown in U.S. Pat. No. 4,129,863 to Gray et al., in U.S. Pat. No. 4,709,225 to Welland et al., in U.S. Pat. No. 5,006,853 to Kiriaki, and in Lee et al., "A Self-Calibrating 15 Bit CMOS A/D Converter," IEEE JSSC, December 1984, p. 813-819. Switched capacitor approaches generally provide good temperature drift and aging characteristics.
Another type of successive approximation ADC is a switched resistor capacitor array ADC. Switched resistor capacitor array ADCs are known in the art as shown in Fotouhi, "High-Resolution Successive Approximation Analog To Digital Conversion Techniques In MOS Integrated Circuits" Dissertation, University of California, 1980, p. 86-93. The switched resistor capacitor array ADC, however, suffers from inaccuracies in the resistor array, resistor temperature drift, and resistor aging drift, all of which may be substantial.